Various methods of developing a latent image have been described in the art of electrophotographic printing and copying systems. Of particular interest with respect to the present invention is the concept of splitting a thin layer of liquid developing material into image and background portions such as the processes disclosed in U.S. Pat. No. 5,826,147 and U.S. Pat. No. 5,937,243, the disclosures of which are totally incorporated herein by reference. In this process, a thin and substantially uniform layer of high concentration liquid developing material is laid onto a latent image bearing surface. A second latent image is created in the toner layer in response to the original latent image. With the latent image bearing toner layer being brought into contact with a separator member, wherein development of the latent image occurs upon separation of the first and second surfaces, as a function of the electric force strength generated by the latent image. In this process, toner particle migration or electrophoresis is replaced by direct surface-to-surface transfer of a toner layer induced by image-wise forces. For the present description, the concept of latent image development via direct surface-to-surface transfer of a toner layer via image-wise forces will be identified generally as Contact Electrostatic Printing (CEP).
One of the embodiments of the CEP process calls for the deposition of a uniform layer of charged marking particles (also referred herein as an ink cake film) on a photoreceptor that has been image-wise exposed. There is a general concern about the uniformity of the ink cake film due to the existence of the latent image. To overcome this non-uniformity problem, there is generally required the application of a very high voltage on the ink cake film donor roll. The voltage on the donor roll, however, is limited by air breakdown in the nip exit due to Paschen breakdown which will damage or destroy the latent image. It would be desirable to have a photoreceptor that has been exposed to light not undergo substantial discharge until after the ink cake film has been applied in order to achieve both ink cake uniformity and latent image fidelity. The present inventors have discovered new photoreceptors and new methods for their preparation wherein the photoreceptor that has been exposed to light does not undergo substantial discharge until after the ink cake film has been applied. The delayed discharge is to be distinguished from the traditional supply limited discharge and the S shaped discharge (also called induction period discharge).
In the traditional discharge depicted in FIG. 1, the supply of carriers from the the generator layer into the transport layer controls the shape of the discharge. The supply efficiency (charges injected into the transport layer per photon absorbed in the generator layer) is a product of the photogeneration efficiency and the injection (from the generator layer into the transport layer) efficiency. The amount of charge neutralized on the surface as measured by the voltage across the photoconducting layers is equal to the charges supplied from the generator layer into the transport layer. The photodischarge curve is linear with a negative slope from the maximum (dark or zero exposure) to the minimum voltage. In such supply limited discharge, the ideal discharge is a linear discharge down to zero or residual voltage with the slope being a measure of the photosensitivity. However, since the photogeneration rate and injection rate in practical materials is electric field dependent and decreasing with field, the discharge slope decreases and the discharge curve at low voltages increasingly departs from the linear discharge.
The S shaped discharge (depicted in FIG. 2) employed in the digital systems is generated by fabricating a particle contact layer in one embodiment of which photocoductor particles are dispersed in insulating binders. The concentration of the charge generating and transporting pigment particles is high enough to maintain particle contact and thus a conducting path through the layer. The key to this S shaped photodischarge is a heterogeneous structure which provides a connected but convoluted path for charge transport or conduction. At high electric fields, after the sample is charged, any charge photogenerated at the surface is directed in a straight line through the layer, encounters a barrier in the insulating region and causes negligible voltage discharge. After nearly all the surface charge is injected, the local electric field normal to the surface is negligible and the remaining charge is able to move in other directions and follow the connected path to a depth below where the initial charge was stopped. At this deeper level the charge again sees the full electric field and encounters the insulating barrier. But because the motion of the previous charge reduced the electric field in the first level, more charge follows the convoluted path down to the next level. Thus by such a cascade, total discharge occurs after a light exposure corresponding to the generation of enough charge required for total discharge, resulting in a step like or S shaped discharge curve. In this S shaped discharge, the induction period is not a time effect but a photon flux effect (as a function of the number of photons in the flash) whereas the delayed discharge (depicted in FIG. 3) discussed in this invention is delayed in time after exposure.
Conventional photoreceptors are disclosed in Takai, U.S. Pat. No. 4,727,009; Kan et al., U.S. Pat. No. 4,784,928; Champ et al., U.S. Pat. No. 4,889,784; Gruenbaum et al., U.S. Pat. No. 5,468,583; Yuh et al., U.S. Pat. No. 5,028,502; Yanus et al., U.S. Pat. 4,806,443; and Yanus et al., U.S. Pat. No. 4,806,444.